A conventional low frequency wide-area lightning detection system detects and locates the return strokes in cloud-to-ground lightning flashes. Although the energy of each return stroke is emitted in a substantially vertical column between cloud and ground, the location of interest for each stroke is the point where the return stroke made contact with the ground. Wide-area lightning detection systems conventionally include many sensors distributed approximately in a grid separated from each other by distances on the order of hundreds of kilometers. Each sensor communicates with a central analyzer so that the signals detected by several sensors may form the basis of a determination of location and current of each return stroke.
A return stroke emits energy that is detected by a sensor in the radio spectrum at comparatively low frequencies of about 1 KHz to about 500 KHz. A return stroke signal in this band of frequencies propagates in the region bounded by the earth's surface and the ionosphere, follows the surface over mountains and valleys, and is generally not obstructed by low terrain or buildings, passing through these obstructions. When a return stroke signal is detected at a sensor, the amplitude of the received signal has been degraded by a combination of physical phenomena. Signal amplitude degrades with distance (i.e., as the crow flies), path length, and conductivity of the terrain. The farther the stroke was from the sensor, the lower the amplitude will be of the received signal. If the distance traveled is over mountainous terrain, the path the signal followed to the sensor may be greater than the path across a smooth earth model; and, therefore the resulting signal amplitude will be still lower. If the conductivity of the terrain is not uniform with distance and bearing to the sensor, inaccurate estimates of amplitude at the location the lightning return stroke occurred will result from use of a smooth ellipsoid, uniformly conducting model of the earth's surface.
Each sensor conventionally detects the time of occurrence of the received signal. When more than one sensor detects a single return stroke, the fact that the return stroke happened at one instant in time can be used to estimate the location of the return stroke and a distance from each sensor to the estimated location of the return stroke. Conventional sensors are synchronized to a common time base so that each can report a time of occurrence of the detected signal. This time is conventionally called a time of arrival. The time of occurrence of the received signal generally suggests a location where the return stroke occurred at the ground.
The antenna or radio spectrum antennas used by a sensor may include omnidirectional antennas and directional antennas. When directional antennas are used, the sensor calculates bearing to the source of the received signal. The bearing generally suggests a location where the return stroke occurred at the ground.
Conventional lightning detection systems use bearing and/or time of arrival information from several sensors to estimate a probable location of a return stroke. Bearing information from two or more sensors having directional antennas is sufficient to suggest location. Time of arrival information from three or more sensors is sufficient to suggest a location. A probable location may be estimated by analyzing the suggested locations when more than one set of information is available (e.g., both bearing and time of arrival information, bearing information from more than two sensors, time of arrival information from more than three sensors).
Received signal peak amplitude is generally proportional to the maximum current of the return stroke at the estimated location. Distance, path length, and conductivity, as discussed above, modify (e.g., degrade, reshape, attenuate, or in some cases partially boost) the received signal amplitude in a complex manner and adversely affect the accuracy of estimates of the peak current of the return stroke.
For conventional lightning detection systems, the accuracy of the estimated location of the return stroke and estimated peak current of the stroke is unsatisfactory for many applications. The estimated time of occurrence, location, and peak current of a return stroke are needed for design and maintenance of equipment and buildings (e.g., related to electric power systems or communication), for risk assessment, and for insurance claims against loss caused by lightning. Without the present invention, conventional lightning detection systems provide a median location and time uncertainty on the order of +/−0.5 km and +/−1 μsec, respectively. Peak current estimates are uncertain to +/−up to 30%. Significant economic value can be achieved by reducing these uncertainties, for example, in more economical lightning protection systems for equipment and buildings, more economical equipment and building maintenance, lower insurance premiums, and fewer disputes regarding the cause of losses that may have been due to lightning.